Glass article

ABSTRACT

A glass article includes a glass core layer and a glass cladding layer adjacent to the core layer. A coefficient of thermal expansion (CTE) of the core layer is greater than a CTE of the cladding layer. The core layer has a tensile stress, and the cladding layer has a compressive stress. A retained strength of the glass article is a strength determined after abrasion of an outer surface of the glass article with 1 mL of 90 grit SiC particles for 5 seconds at an abrasion pressure, and a ratio of the retained strength at an abrasion pressure of 25 psi to the retained strength at an abrasion pressure of 5 psi is at least about 0.3.

This application is a divisional of U.S. patent application Ser. No.15/129,278 filed on Sep. 26, 2016, which claims the benefit of priorityunder 35 U.S.C. § 371 of International Application No. PCT/US2015/022413filed on Mar. 25, 2015, which claims the benefit of priority to U.S.Application No. 61/970,972 filed on Mar. 27, 2014 the content of each ofwhich is incorporated herein by reference in its entirety.

BACKGROUND 1. Field

This disclosure relates to glass articles, and more particularly tolaminated glass articles comprising a plurality of glass layers andmethods for forming the same.

2. Technical Background

Glass articles can be used in a wide variety of products including, forexample, auto-glazing, architectural panels, appliances, and cover glass(e.g., for touch-screen devices such as smartphones, tablets, laptopcomputers, and monitors). Relatively large flaws can be introduced intothe surfaces of glass articles during use. For example, it has beenobserved that flaws as deep as 200 μm or more have been introduced intothe surface of automotive windshields. Thus, it would be desirable forglass articles to have high scratch resistance and to retain theirstrength even after introduction of relatively large flaws into thesurfaces thereof.

SUMMARY

Disclosed herein is a glass article comprising a glass core layer and aglass cladding layer adjacent to the core layer. A coefficient ofthermal expansion (CTE) of the core layer is greater than a CTE of thecladding layer. The core layer comprises a tensile stress, and thecladding layer comprises a compressive stress. A retained strength ofthe glass article comprises a strength determined after abrasion of anouter surface of the glass article with 1 mL of 90 grit SiC particlesfor 5 seconds at an abrasion pressure, and a ratio of the retainedstrength at an abrasion pressure of 25 psi to the retained strength atan abrasion pressure of 5 psi is at least about 0.3.

Also disclosed herein is a method comprising forming a glass articlecomprising a core layer and a cladding layer adjacent to the core layer.A coefficient of thermal expansion (CTE) mismatch between the core layerand the cladding layer and a ratio of a thickness of the core layer to athickness of the glass article are selected such that a Knoop scratchthreshold of the glass article is at least about 5 N, an indentationthreshold of the glass article is at least about 20 N, and a compressivestress of the cladding layer is at most about 500 MPa.

Also disclosed herein is a glass article comprising a glass core layerand a glass cladding layer adjacent to the core layer. The core layercomprises from about 50 mol % to about 80 mol % SiO₂, from about 5 mol %to about 20 mol % Al₂O₃, and from about 1 mol % to about 8 mol % MgO.The cladding layer comprises from about 60 mol % to about 70 mol % SiO₂,from about 6 mol % to about 18 mol % Al₂O₃, from about 4 mol % to about21 mol % B₂O₃, from about 0.2 mol % to about 5 mol % MgO, and from about3 mol % to about 12 mol % CaO. A Knoop scratch threshold of the glassarticle is at least about 5 N, and an indentation threshold of the glassarticle is at least about 20 N.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theembodiments as described herein, including the detailed descriptionwhich follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understanding the natureand character of the claims. The accompanying drawings are included toprovide a further understanding, and are incorporated in and constitutea part of this specification. The drawings illustrate one or moreembodiment(s), and together with the description serve to explainprinciples and operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of one exemplary embodiment of a glassarticle.

FIG. 2 is a cross-sectional view of one exemplary embodiment of anoverflow distributor that can be used to form a glass article.

FIG. 3 is a graphical illustration of the retained strength vs. abrasionpressure for laminated glass sheets formed according to Example 1 andnon-strengthened glass sheets formed according to Comparative Example 1.

FIG. 4 is a graphical illustration of the retained strength vs. abrasionpressure for laminated glass sheets formed according to Example 2 andion-exchanged glass sheets formed according to Comparative Example 2.

FIG. 5 is a graphical illustration of the retained strength vs. abrasionpressure for laminated glass sheets formed according to Example 2 andion-exchanged glass sheets formed according to Comparative Example 3.

FIG. 6 is a graphical illustration of the retained strength vs. abrasionpressure for laminated glass sheets formed according to Example 3,non-strengthened glass sheets formed according to Comparative Example 1,and ion-exchanged glass sheets formed according to Comparative Example3.

FIG. 7 is a graphical illustration of the retained strength (given asload to failure) vs. ramped Knoop scratch load for laminated glasssheets formed according to Example 3 and heat-strengthened glass sheetsformed according to Comparative Example 4.

FIG. 8 is a graphical illustration of the retained strength vs. rampedscratch load for laminated glass sheets formed according to Example 3,heat-strengthened glass sheets formed according to Comparative Example4, and fully-tempered glass sheets formed according to ComparativeExample 5.

FIG. 9 is a graphical illustration of the indentation thresholds oflaminated glass sheets formed according to Examples 2 and 3,non-strengthened glass sheets formed according to Comparative Example 1,and ion-exchanged glass sheets formed according to Comparative Examples2, 3, 6, and 7.

FIG. 10 is a graphical illustration of the indentation thresholds oflaminated glass sheets formed according to Examples 4 and 5,non-strengthened glass sheets formed according to Comparative Example 1,and ion-exchanged glass sheets formed according to Comparative Examples2, 3, 6, and 7.

FIG. 11 is a graphical illustration of the Knoop scratch thresholds oflaminated glass sheets formed according to Examples 2 and 3,non-strengthened glass sheets formed according to Comparative Example 1,and ion-exchanged glass sheets formed according to Comparative Examples2, 3, 6, and 7.

FIG. 12 is a graphical illustration of the Vickers scratch thresholds oflaminated glass sheets formed according to Examples 2 and 3,non-strengthened glass sheets formed according to Comparative Example 1,and ion-exchanged glass sheets formed according to Comparative Examples2, 3, and 7.

FIG. 13 is a graphical illustration of the drop thresholds of laminatedglass sheets formed according to Example 3 and ion-exchanged glasssheets formed according to Comparative Examples 8 and 9.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments which areillustrated in the accompanying drawings. Whenever possible, the samereference numerals will be used throughout the drawings to refer to thesame or like parts. The components in the drawings are not necessarilyto scale, emphasis instead being placed upon illustrating the principlesof the exemplary embodiments.

As used herein, the term “average coefficient of thermal expansion”refers to the average coefficient of thermal expansion of a givenmaterial or layer between 0° C. and 300° C. As used herein, the term“coefficient of thermal expansion” refers to the average coefficient ofthermal expansion unless otherwise indicated.

The strengths of the glass articles described herein are determinedusing ring-on-ring loading according to the test method described inASTM C1499-08, Standard Test Method for Monotonic Equibiaxial FlexuralStrength of Advanced Ceramics at Ambient Temperature. Generally, thering-on-ring loading test method is used to determine the biaxialstrength of advanced brittle materials at ambient temperature viaconcentric ring configurations under monotonic uniaxial loading, and hasbeen widely accepted as a method for evaluating the surface strength ofglass articles. The ring-on-ring loading results described herein aredetermined using a 1 in diameter support ring and a 0.5 in diameterloading ring on a 2 in square glass sheet. The contact radius of thering is 1.6 mm, and the head speed is 1.2 mm/min.

As used herein, the term “retained strength” refers to the strength of aglass article determined after controlled introduction of flaws to anouter surface of the glass article. In some embodiments, the flaws areintroduced by abrading the outer surface of the glass article with 1 mLof 90 grit SiC particles for 5 seconds at an abrasion pressure. Suchretained strength values generally are reported herein at abrasionpressures of 5 psi, 15 psi, and 25 psi. In other embodiments, the flawsare introduced by scratching the outer surface of the glass article witha Knoop diamond at increasing ramped scratch loads. Such retainedstrength values generally are reported herein at ramped scratch loads of0 to 3 N, 0 to 7 N, and 0 to 10 N.

As used herein, the term “Knoop scratch threshold” refers to the load atwhich lateral cracking is first observed in a glass article in responseto scratching the surface of the glass article with a Knoop diamond atincreasing load. A mechanical tester holds the Knoop diamond with whichthe glass article is scratched at increasing loads to determine theonset of lateral cracking. The lateral cracking is evidenced bysustained cracks in the glass article that are greater than twice thewidth of the original scratch or groove formed by the Knoop diamond. Thetest is performed at room temperature in 50% relative humidity.

As used herein, the term “indentation threshold” refers to the load atwhich cracks are first observed in a glass article in response toindentation of the surface of the glass article with a Vickers indenterat increasing load. An indentation load is applied to and then removedfrom a surface of the glass article with the Vickers indenter at a rateof 0.2 mm/min. The maximum indentation load is held for 10 seconds. Theindentation threshold is determined as the indentation load at which 50%of 10 indents exhibit any number of radial/median cracks emanating fromthe corners of the indent impression. The maximum indentation load isincreased until the indentation threshold is met for a given glassarticle. All indentation measurements are performed at room temperaturein 50% relative humidity.

As used herein, the term “Vickers scratch threshold” refers to the loadat which lateral cracking is first observed in a glass article inresponse to scratching the surface of the glass article with a Vickersindenter at increasing load. The test procedure is similar to that usedto determine Knoop scratch threshold, except that the Vickers indenteris substituted for the Knoop diamond. The lateral cracking is evidencedby sustained cracks in the glass article that are greater than twice thewidth of the original scratch or groove formed by the Vickers indenter.

As used herein, the term “drop threshold” refers to a height at whichfailure of a cover glass is first observed in response to dropping anelectronic device comprising the cover glass onto a drop surface fromincreasing heights. The drop threshold values described herein generallywere determined using a case from a commercially available smartphoneretrofitted with the cover glass as the electronic device and a LeChaseA asphalt drop surface.

In various embodiments, a glass article comprises at least a first layerand a second layer. For example, the first layer comprises a core layer,and the second layer comprises one or more cladding layers adjacent tothe core layer. The first layer and/or the second layer are glass layerscomprising a glass, a glass-ceramic, or a combination thereof. In someembodiments, the first layer and/or the second layer are transparentglass layers.

FIG. 1 is a cross-sectional view of one exemplary embodiment of a glassarticle 100. In some embodiments, glass article 100 comprises alaminated sheet comprising a plurality of glass layers. The laminatedsheet can be substantially planar as shown in FIG. 1 or non-planar.Glass article 100 comprises a core layer 102 disposed between a firstcladding layer 104 and a second cladding layer 106. In some embodiments,first cladding layer 104 and second cladding layer 106 are exteriorlayers as shown in FIG. 1. In other embodiments, the first claddinglayer and/or the second cladding layer are intermediate layers disposedbetween the core layer and an exterior layer.

Core layer 102 comprises a first major surface and a second majorsurface opposite the first major surface. In some embodiments, firstcladding layer 104 is fused to the first major surface of core layer102. Additionally, or alternatively, second cladding layer 106 is fusedto the second major surface of core layer 102. In such embodiments, theinterfaces between first cladding layer 104 and core layer 102 and/orbetween second cladding layer 106 and core layer 102 are free of anybonding material such as, for example, an adhesive, a coating layer, orany non-glass material added or configured to adhere the respectivecladding layers to the core layer. Thus, first cladding layer 104 and/orsecond cladding layer 106 are fused directly to core layer 102 or aredirectly adjacent to core layer 102. In some embodiments, the glassarticle comprises one or more intermediate layers disposed between thecore layer and the first cladding layer and/or between the core layerand the second cladding layer. For example, the intermediate layerscomprise intermediate glass layers and/or diffusions layers formed atthe interface of the core layer and the cladding layer. In someembodiments, glass article 100 comprises a glass-glass laminate in whichthe interfaces between directly adjacent glass layers are glass-glassinterfaces.

In some embodiments, core layer 102 comprises a first glass composition,and first and/or second cladding layers 104 and 106 comprise a secondglass composition that is different than the first glass composition.For example, in the embodiment shown in FIG. 1, core layer 102 comprisesthe first glass composition, and each of first cladding layer 104 andsecond cladding layer 106 comprises the second glass composition. Inother embodiments, the first cladding layer comprises the second glasscomposition, and the second cladding layer comprises a third glasscomposition that is different than the first glass composition and/orthe second glass composition.

The glass article can be formed using a suitable process such as, forexample, a fusion draw, down draw, slot draw, up draw, or float process.In some embodiments, the glass article is formed using a fusion drawprocess. FIG. 2 is a cross-sectional view of one exemplary embodiment ofan overflow distributor 200 that can be used to form a glass articlesuch as, for example, glass article 100. Overflow distributor 200 can beconfigured as described in U.S. Pat. No. 4,214,886, which isincorporated herein by reference in its entirety. For example, overflowdistributor 200 comprises a lower overflow distributor 220 and an upperoverflow distributor 240 positioned above the lower overflowdistributor. Lower overflow distributor 220 comprises a trough 222. Afirst glass composition 224 is melted and fed into trough 222 in aviscous state. First glass composition 224 forms core layer 102 of glassarticle 100 as further described below. Upper overflow distributor 240comprises a trough 242. A second glass composition 244 is melted and fedinto trough 242 in a viscous state. Second glass composition 244 formsfirst and second cladding layers 104 and 106 of glass article 100 asfurther described below.

First glass composition 224 overflows trough 222 and flows down opposingouter forming surfaces 226 and 228 of lower overflow distributor 220.Outer forming surfaces 226 and 228 converge at a draw line 230. Theseparate streams of first glass composition 224 flowing down respectiveouter forming surfaces 226 and 228 of lower overflow distributor 220converge at draw line 230 where they are fused together to form corelayer 102 of glass article 100.

Second glass composition 244 overflows trough 242 and flows downopposing outer forming surfaces 246 and 248 of upper overflowdistributor 240. Second glass composition 244 is deflected outward byupper overflow distributor 240 such that the second glass compositionflows around lower overflow distributor 220 and contacts first glasscomposition 224 flowing over outer forming surfaces 226 and 228 of thelower overflow distributor. The separate streams of second glasscomposition 244 are fused to the respective separate streams of firstglass composition 224 flowing down respective outer forming surfaces 226and 228 of lower overflow distributor 220. Upon convergence of thestreams of first glass composition 224 at draw line 230, second glasscomposition 244 forms first and second cladding layers 104 and 106 ofglass article 100.

In some embodiments, first glass composition 224 of core layer 102 inthe viscous state is contacted with second glass composition 244 offirst and second cladding layers 104 and 106 in the viscous state toform the laminated sheet. In some of such embodiments, the laminatedsheet is part of a glass ribbon traveling away from draw line 230 oflower overflow distributor 220 as shown in FIG. 2. The glass ribbon canbe drawn away from lower overflow distributor 220 by a suitable meansincluding, for example, gravity and/or pulling rollers. The glass ribboncools as it travels away from lower overflow distributor 220. The glassribbon is severed to separate the laminated sheet therefrom. Thus, thelaminated sheet is cut from the glass ribbon. The glass ribbon can besevered using a suitable technique such as, for example, scoring,bending, thermally shocking, and/or laser cutting. In some embodiments,glass article 100 comprises the laminated sheet as shown in FIG. 1. Inother embodiments, the laminated sheet can be processed further (e.g.,by cutting or molding) to form glass article 100.

Although glass article 100 shown in FIG. 1 comprises three layers, otherembodiments are included in this disclosure. In other embodiments, aglass article can have a determined number of layers, such as two, four,or more layers. For example, a glass article comprising two layers canbe formed using two overflow distributors positioned so that the twolayers are joined while traveling away from the respective draw lines ofthe overflow distributors or using a single overflow distributor with adivided trough so that two glass compositions flow over opposing outerforming surfaces of the overflow distributor and converge at the drawline of the overflow distributor. A glass article comprising four ormore layers can be formed using additional overflow distributors and/orusing overflow distributors with divided troughs. Thus, a glass articlehaving a determined number of layers can be formed by modifying theoverflow distributor accordingly.

In some embodiments, glass article 100 comprises a thickness of at leastabout 0.05 mm, at least about 0.1 mm, at least about 0.2 mm, or at leastabout 0.3 mm. Additionally, or alternatively, glass article 100comprises a thickness of at most about 2 mm, at most about 1.5 mm, atmost about 1 mm, at most about 0.7 mm, or at most about 0.5 mm. In someembodiments, a ratio of a thickness of core layer 102 to a thickness ofglass article 100 is at least about 0.8, at least about 0.85, at leastabout 0.9, or at least about 0.95. In some embodiments, a thickness ofthe second layer (e.g., each of first cladding layer 104 and secondcladding layer 106) is from about 0.01 mm to about 0.3 mm.

In some embodiments, the first glass composition and/or the second glasscomposition comprise a liquidus viscosity suitable for forming glassarticle 100 using a fusion draw process as described herein. Forexample, the first glass composition of the first layer (e.g., corelayer 102) comprises a liquidus viscosity of at least about 100 kP, atleast about 200 kP, or at least about 300 kP. Additionally, oralternatively, the first glass composition comprises a liquidusviscosity of at most about 3000 kP, at most about 2500 kP, at most about1000 kP, or at most about 800 kP. Additionally, or alternatively, thesecond glass composition of the second layer (e.g., first and/or secondcladding layers 104 and 106) comprises a liquidus viscosity of at leastabout 50 kP, at least about 100 kP, or at least about 200 kP.Additionally, or alternatively, the second glass composition comprises aliquidus viscosity of at most about 3000 kP, at most about 2500 kP, atmost about 1000 kP, or at most about 800 kP. The first glass compositioncan aid in carrying the second glass composition over the overflowdistributor to form the second layer. Thus, the second glass compositioncan comprise a liquidus viscosity that is lower than generallyconsidered suitable for forming a single layer sheet using a fusion drawprocess.

In some embodiments, glass article 100 is configured as a strengthenedglass article. For example, in some embodiments, the second glasscomposition of the second layer (e.g., first and/or second claddinglayers 104 and 106) comprises a different average coefficient of thermalexpansion (CTE) than the first glass composition of the first layer(e.g., core layer 102). For example, first and second cladding layers104 and 106 are formed from a glass composition having a lower averageCTE than core layer 102. The CTE mismatch (i.e., the difference betweenthe average CTE of first and second cladding layers 104 and 106 and theaverage CTE of core layer 102) results in formation of compressivestress in the cladding layers and tensile stress in the core layer uponcooling of glass article 100. In various embodiments, each of the firstand second cladding layers, independently, can have a higher averageCTE, a lower average CTE, or substantially the same average CTE as thecore layer.

In some embodiments, the average CTE of the first layer (e.g., corelayer 102) and the average CTE of the second layer (e.g., first and/orsecond cladding layers 104 and 106) differ by at least about 5×10⁻⁷°C.⁻¹, at least about 15×10⁻⁷° C.⁻¹, or at least about 25×10⁻⁷° C.⁻¹.Additionally, or alternatively, the average CTE of the first layer andthe average CTE of the second layer differ by at most about 55×10⁻⁷°C.⁻¹, at most about 50×10⁻⁷° C.⁻¹, at most about 40×10⁻⁷° C.⁻¹, at mostabout 30×10⁻⁷° C.⁻¹, at most about 20×10⁻⁷° C.⁻¹, or at most about10×10⁻⁷° C.⁻¹. For example, in some embodiments, the average CTE of thefirst layer and the average CTE of the second layer differ by from about5×10⁻⁷° C.⁻¹ to about 30×10⁻⁷° C.⁻¹ or from about 5×10⁻⁷° C.⁻¹ to about20×10⁻⁷° C.⁻¹. In some embodiments, the second glass composition of thesecond layer comprises an average CTE of at most about 40×10⁻⁷° C.⁻¹, orat most about 35×10⁻⁷° C.⁻¹. Additionally, or alternatively, the secondglass composition of the second layer comprises an average CTE of atleast about 25×10⁻⁷° C.⁻¹, or at least about 30×10⁻⁷° C.⁻¹.Additionally, or alternatively, the first glass composition of the firstlayer comprises an average CTE of at least about 40×10⁻⁷° C.⁻¹, at leastabout 50×10⁻⁷° C.⁻¹, or at least about 55×10⁻⁷° C.⁻¹. Additionally, oralternatively, the first glass composition of the first layer comprisesan average CTE of at most about 90×10⁻⁷° C.⁻¹, at most about 85×10⁻⁷°C.⁻¹, at most about 80×10⁻⁷° C.⁻¹, at most about 70×10⁻⁷° C.⁻¹, or atmost about 60×10⁻⁷° C.⁻¹.

In various embodiments, the glass compositions and the relativethicknesses of the glass layers can be selected to achieve a glassarticle having desired strength properties. For example, in someembodiments, the first glass composition of the first layer (e.g., corelayer 102) and the second glass composition of the second layer (e.g.,first and/or second cladding layers 104 and 106) are selected to achievea desired CTE mismatch, and the thicknesses of each of the first layerand the second layer are selected, in combination with the desired CTEmismatch, to achieve a desired compressive stress in the second layer, adesired tensile stress in the first layer, a desired retained strength,and/or a desired drop threshold.

In various embodiments, the glass compositions and the relativethicknesses of the glass layers can be selected to achieve a glassarticle having desired surface properties. For example, in someembodiments, the first glass composition of the first layer (e.g., corelayer 102), the second glass composition of the second layer (e.g.,first and/or second cladding layers 104 and 106), and the thicknesses ofeach of the first layer and the second layer are selected to achieve aglass article having a desired Knoop scratch threshold and/or a desiredindentation threshold.

In some embodiments, the Knoop scratch threshold of the glass article isat least about 5 N, at least about 10 N, or at least about 15 N.Additionally, or alternatively, the indentation threshold of the glassarticle is at least about 20 N, at least about 30 N, or at least about40 N. Additionally, or alternatively, the Vickers scratch threshold ofthe glass article is at least about 2 N, at least about 3 N, at leastabout 5 N, or at least about 7 N. Additionally, or alternatively, thedrop threshold of the glass article is at least about 100 cm, at leastabout 140 cm, or at least about 160 cm.

The strength and surface properties of the glass article can be relatedto one another. Thus, the various properties of the glass article (e.g.,CTE mismatch, compressive stress, tensile stress, retained strength,drop threshold, Knoop scratch threshold, and/or indentation threshold)generally cannot be adjusted independently of one another, but rather,are adjusted in combination or balanced with one another to produce theglass article. Conventional wisdom suggests that a glass article withrelatively higher compressive stress will have higher Knoop scratch,indentation, and drop threshold values than a glass article withrelatively lower compressive stress. However, proper selection of thevarious properties as described herein can enable formation of a glassarticle having improved Knoop scratch, indentation, and/or dropthresholds with relatively lower compressive stress. The improvedproperties can be imparted to the glass article as-formed withoutadditional processing (e.g., ion-exchange treatment). For example, theas-formed fusion formed glass laminate can have improved Knoop scratch,indentation, and/or drop thresholds compared to ion-exchanged glassarticles with higher compressive stresses than the fusion formed glasslaminate.

In some embodiments, the compressive stress of the cladding layers is atmost about 800 MPa, at most about 500 MPa, at most about 300 MPa, atmost about 200 MPa, at most about 150 MPa, at most about 100 MPa, atmost about 50 MPa, or at most about 40 MPa. Additionally, oralternatively, the compressive stress of the cladding layers is at leastabout 10 MPa, at least about 20 MPa, at least about 30 MPa, at leastabout 50 MPa, or at least about 100 MPa.

The glass articles described herein can be useful in a variety ofapplications including, for example, auto-glazing, architectural,appliance, and consumer electronics (e.g., cover glass) applications.Glass articles with relatively lower strength can be beneficial for use,for example, in automotive applications (e.g., automotive windshields).Glass articles with relatively higher strength can be beneficial foruse, for example, in cover glass applications (e.g., touchscreendevices). In some embodiments, the glass compositions and the relativethicknesses of the glass layers are selected as described herein to forma glass article having a predetermined strength (e.g., unabradedstrength and/or retained strength). Thus, the strength of the glassarticle is controlled by manipulating the glass compositions andrelative thicknesses of the glass layers to form a glass article that issuitable for a desired application.

In various applications, it can be beneficial for the strength of theglass article to remain substantially constant in response to varyingdegrees of surface abrasion or surface insults. In other words, it canbe beneficial for the strength of the glass article to be relativelyinsensitive to surface abrasion or surface insults. Thus, the strengthof the glass article can remain substantially constant over the life ofthe glass article, even after varying amounts of damage are caused tothe surface of the glass article during use thereof. The strength of theglass article can remain at or near the predetermined strength duringthe life of the glass article.

In some embodiments, a ratio of the retained strength of the glassarticle at an abrasion pressure of 25 psi to the retained strength ofthe glass article at an abrasion pressure of 5 psi is at least about0.3, at least about 0.4, at least about 0.5, at least about 0.6, atleast about 0.7, at least about 0.8, or at least about 0.9.Additionally, or alternatively, a ratio of the retained strength of theglass article at a ramped Knoop scratch load of 0 to 10 N to theretained strength of the glass article at a ramped Knoop scratch load of0 to 3 N is at least about 0.3, at least about 0.4, at least about 0.5,at least about 0.6, at least about 0.7, at least about 0.8, or at leastabout 0.9. Additionally, or alternatively, each of the retained strengthat an abrasion pressure of 5 psi and the retained strength at anabrasion pressure of 25 psi is at least about 80 MPa, at least about 90MPa, or at least about 100 MPa. Additionally, or alternatively, each ofthe retained strength at an abrasion pressure of 5 psi and the retainedstrength at an abrasion pressure of 25 psi is at most about 375 MPa, atmost about 350 MPa, or at most about 300 MPa.

In some embodiments, the glass article comprises an elastic propertygradient. Thus, one or more elastic properties of the glass article varyalong the thickness of the glass article. For example, one or more ofthe Young's modulus, the shear modulus, or the Poisson's ratio of theglass article varies in the thickness direction of the glass article.The elastic property gradient can be formed, for example, as a result ofdifferent glass layers having different elastic properties and/orinterdiffusion between adjacent glass layers. In some embodiments, theelastic property gradient comprises a step change in the elasticproperty at the interface between the first layer and the second layerof the glass article (e.g., the clad/core interface). For example, atleast one elastic property of the first layer is different than thecorresponding elastic property of the second layer. In otherembodiments, the elastic property gradient comprises a gradual variationin which at least one elastic property changes from a first value at afirst position along the thickness direction of the glass article to asecond value at a second position along the thickness direction of theglass article. For example, the at least one elastic property of thesecond layer decreases in a direction away from the core layer toward asurface of the glass article. The elastic property gradient can enhancethe damage resistance of the glass article. For example, the gradientregion can be on the order of an indent size (e.g., tens of microns) toenhance the damage resistance of the glass article.

The first glass composition of the first layer (e.g., core layer 102)and the second glass composition of the second layer (e.g., firstcladding layer 104 and/or second cladding layer 106) can comprisesuitable glass compositions capable of forming a glass article withdesired properties as described herein. Exemplary first (core) glasscompositions and exemplary second (clad) glass compositions are shown inTables 1 and 2, respectively. The amounts of the various components aregiven in Tables 1 and 2 as mol % on an oxide basis.

In some embodiments, the first glass composition comprises a glassnetwork former selected from the group consisting of SiO₂, Al₂O₃, B₂O₃,and combinations thereof. For example, the first glass compositioncomprises at least about 50 mol % SiO₂, at least about 55 mol % SiO₂, atleast about 60 mol % SiO₂, or at least about 65 mol % SiO₂.Additionally, or alternatively, the first glass composition comprises atmost about 80 mol % SiO₂, at most about 70 mol % SiO₂, at most about 68mol % SiO₂, or at most about 60 mol % SiO₂. Additionally, oralternatively, the first glass composition comprises at least about 5mol % Al₂O₃, at least about 9 mol % Al₂O₃, or at least about 12 mol %Al₂O₃. Additionally, or alternatively, the first glass compositioncomprises at most about 20 mol % Al₂O₃, at most about 17 mol % Al₂O₃, orat most about 11 mol % Al₂O₃. Additionally, or alternatively, the firstglass composition comprises at least about 3 mol % B₂O₃, at least about6 mol % B₂O₃, or at least about 7 mol % B₂O₃. Additionally, oralternatively, the first glass composition comprises at most about 11mol % B₂O₃, at most about 8 mol % B₂O₃, or at most about 4 mol % B₂O₃.In some embodiments, the first glass composition is substantially freeof B₂O₃. For example, the first glass composition comprises at mostabout 0.1 mol % B₂O₃.

In some embodiments, the first glass composition comprises an alkalimetal oxide selected from the group consisting of Li₂O, Na₂O, K₂O, andcombinations thereof. For example, the first glass composition comprisesat least about 0.05 mol % Na₂O, at least about 10 mol % Na₂O, or atleast about 13 mol % Na₂O. Additionally, or alternatively, the firstglass composition comprises at most about 16 mol % Na₂O, at most about14 mol % Na₂O, at most about 2 mol % Na₂O, or at most about 0.1 mol %Na₂O. Additionally, or alternatively, the first glass compositioncomprises at least about 0.01 mol % K₂O, at least about 2 mol % K₂O, orat least about 8 mol % K₂O. Additionally, or alternatively, the firstglass composition comprises at most about 15 mol % K₂O, at most about 9mol % K₂O, at most about 6 mol % K₂O, or at most about 0.1 mol % K₂O.

In some embodiments, the first glass composition comprises an alkalineearth oxide selected from the group consisting of MgO, CaO, SrO, BaO,and combinations thereof. For example, the first glass compositioncomprises at least about 1 mol % MgO, at least about 2 mol % MgO, atleast about 3 mol % MgO, or at least about 4 mol % MgO. Additionally, oralternatively, the first glass composition comprises at most about 8 mol% MgO, at most about 4 mol % MgO, or at most about 3 mol % MgO.Additionally, or alternatively, the first glass composition comprises atleast about 0.01 mol % CaO, at least about 2 mol % CaO, at least about 4mol % CaO, at least about 5 mol % CaO, or at least about 6 mol % CaO.Additionally, or alternatively, the first glass composition comprises atmost about 8 mol % CaO, at most about 7 mol % CaO, at most about 0.1 mol% CaO, or at most about 0.01 mol % CaO. Additionally, or alternatively,the first glass composition comprises at least about 3 mol % SrO, atleast about 4 mol % SrO, at least about 5 mol % SrO, or at least about 6mol % SrO. Additionally, or alternatively, the first glass compositioncomprises at most about 7 mol % SrO, at most about 6 mol % SrO, or atmost about 5 mol % SrO. Additionally, or alternatively, the first glasscomposition comprises at least about 0.01 mol % BaO, at least about 0.02mol % BaO, or at least about 0.07 mol % BaO. Additionally, oralternatively, the first glass composition comprises at most about 0.1mol % BaO, at most about 0.09 mol % BaO, at most about 0.05 mol % BaO,or at most about 0.01 mol % BaO. In some embodiments, the first glasscomposition is substantially free of SrO. For example, the first glasscomposition comprises at most about 0.1 mol % SrO.

In some embodiments, the first glass composition comprises one or moreadditional components including, for example SnO₂, Sb₂O₃, As₂O₃, Ce₂O₃,CI (e.g., derived from KCl or NaCl), ZrO₂, or Fe₂O₃.

In some embodiments, the second glass composition comprises a glassnetwork former selected from the group consisting of SiO₂, Al₂O₃, B₂O₃,and combinations thereof. For example, the second glass compositioncomprises at least about 60 mol % SiO₂, at least about 62 mol % SiO₂, orat least about 67 mol % SiO₂. Additionally, or alternatively, the secondglass composition comprises at most about 70 mol % SiO₂, at most about68 mol % SiO₂, at most about 65 mol % SiO₂, or at most about 63 mol %SiO₂. Additionally, or alternatively, the second glass compositioncomprises at least about 6 mol % Al₂O₃, at least about 10 mol % Al₂O₃,or at least about 12 mol % Al₂O₃. Additionally, or alternatively, thesecond glass composition comprises at most about 18 mol % Al₂O₃, at mostabout 13 mol % Al₂O₃, or at most about 8 mol % Al₂O₃. Additionally, oralternatively, the second glass composition comprises at least about 4mol % B₂O₃, at least about 6 mol % B₂O₃, at least about 9 mol % B₂O₃, orat least about 16 mol % B₂O₃. Additionally, or alternatively, the secondglass composition comprises at most about 21 mol % B₂O₃, at most about18 mol % B₂O₃, or at most about 11 mol % B₂O₃.

In some embodiments, the second glass composition comprises an alkalimetal oxide selected from the group consisting of Li₂O, Na₂O, K₂O, andcombinations thereof. For example, the second glass compositioncomprises from about 0 mol % to about 0.1 mol % Na₂O, or from about 0mol % to about 0.06 mol % Na₂O. Additionally, or alternatively, thesecond glass composition comprises from about 0 mol % to about 0.05 mol% K₂O, or from about 0 mol % to about 0.03 mol % K₂O. In someembodiments, the second glass composition is substantially free ofalkali metal. For example, the second glass composition comprises atmost about 0.1 mol % alkali metal oxide. In other embodiments, thesecond glass composition comprises from about 5 mol % to about 10 mol %alkali metal oxide.

In some embodiments, the second glass composition comprises an alkalineearth oxide selected from the group consisting of MgO, CaO, SrO, BaO,and combinations thereof. For example, the second glass compositioncomprises at least about 0.2 mol % MgO, at least about 1 mol % MgO, orat least about 3 mol % MgO. Additionally, or alternatively, the secondglass composition comprises at most about 5 mol % MgO, at most about 4mol % MgO, at most about 2 mol % MgO, or at most about 0.5 mol % MgO.Additionally, or alternatively, the second glass composition comprisesat least about 3 mol % CaO, at least about 4 mol % CaO, at least about 5mol % CaO, or at least about 8 mol % CaO. Additionally, oralternatively, the second glass composition comprises at most about 12mol % CaO, at most about 9 mol % CaO, at most about 8 mol % CaO, or atmost about 5 mol % CaO. Additionally, or alternatively, the second glasscomposition comprises at least about 0.2 mol % SrO, at least about 1 mol% SrO, or at least about 2 mol % SrO. Additionally, or alternatively,the second glass composition comprises at most about 3 mol % SrO, atmost about 2 mol % SrO, or at most about 1 mol % SrO. Additionally, oralternatively, the second glass composition comprises at least about0.01 mol % BaO, at least about 0.02 mol % BaO, or at least about 1 mol %BaO. Additionally, or alternatively, the second glass compositioncomprises at most about 2 mol % BaO, at most about 0.5 mol % BaO, atmost about 0.03 mol % BaO, at most about 0.02 mol % BaO, or at mostabout 0.01 mol % BaO. In some embodiments, the second glass compositioncomprises from about 3 mol % to about 16 mol % alkaline earth oxide.

In some embodiments, the second glass composition comprises one or moreadditional components including, for example SnO₂, Sb₂O₃, As₂O₃, Ce₂O₃,CI (e.g., derived from KCl or NaCl), ZrO₂, or Fe₂O₃.

TABLE 1 Exemplary First (Core) Glass Compositions 1-1 1-2 1-3 1-4 1-51-6 SiO₂ 61.97 66.54 63.46 69.05 56.12 59.05 Al₂O₃ 10.89 10.03  9.5610.2  16.73 15.1 B₂O₃ 10.09 6  7.09 10.5 6.26 Na₂O  0.07 0.09  0.0915.13 0.064 0.086 K₂O  2.17 5.79  5.79 3.084 8.059 MgO  6.16 1.9  2.49 5.49 3.74 1.13 CaO  5.45 6.23  7.41 4.61 5.16 SrO  3.09 3.28  3.95 4.835.02 BaO  0.03 0.04  0.04 0.08 0.08 SnO₂  0.04 0.07  0.07  0.13 0.0920.151 ZrO₂  0.01 0.02  0.03 0.023 0.032 Fe₂O₃  0.01 0.01  0.01 0.0330.031 As₂O₃ 0.0002 0.0002 1-7 1-8 1-9 1-10 1-11 SiO₂ 56.24 67.45 69.1768.84 69.86 Al₂O₃ 14.38 12.69 8.53 10.63 10.99 B₂O₃ 7.16 3.67 Na₂O 0.08413.67 13.94 14.86 11.74 K₂O 8.042 0.02 1.17 0.02 4.07 MgO 1.48 2.36 6.455.43 2.87 CaO 6.13 0.03 0.54 0.04 SrO 6.04 BaO 0.1 0.1 SnO₂ 0.164 0.090.19 0.17 0.45 ZrO₂ 0.051 0.01 Fe₂O₃ 0.032 0.01 As₂O 0.0002

TABLE 2 Exemplary Second (Clad) Glass Compositions 2-1 2-2 2-3 2-4 2-52-6 2-7 2-8 SiO₂ 69.44 67.55 62.93 64.59 60.48 62.39 67.29 60.19 Al₂O₃12.36 11 10.74 7.38 11.55 17.21 6.47 11.66 B₂O₃ 4.35 9.83 13.16 16.4517.6 10.5 20.99 17.75 Na₂O 0.05 K₂O 0.022 MgO 4.01 2.26 3.58 2.21 1.381.41 0.35 1.38 CaO 5.98 8.73 7.32 8.14 7.01 7.51 4.49 7.07 SrO 1.73 0.522.17 1.11 1.86 0.83 0.29 1.79 BaO 1.98 0.01 0.01 0.02 0.01 SnO₂ 0.1 0.070.07 0.06 0.208 0.16 0.05 0.16 ZrO₂ 0.03 0.02 0.14 0.05 Fe₂O₃ 0.02 0.010.01 0.01 0.039 0.01 As₂O₃ 0.0004

The glass articles described herein can be used for a variety ofapplications including, for example, for cover glass or glass backplaneapplications in consumer or commercial electronic devices including, forexample, LCD and LED displays, computer monitors, and automated tellermachines (ATMs); for touch screen or touch sensor applications, forportable electronic devices including, for example, mobile telephones,personal media players, and tablet computers; for integrated circuitapplications including, for example, semiconductor wafers; forphotovoltaic applications; for architectural glass applications; forautomotive or vehicular glass applications; or for commercial orhousehold appliance applications.

EXAMPLES

Various embodiments will be further clarified by the following examples.

Example 1

A laminated glass sheet having the general structure shown in FIG. 1 wasformed using a fusion draw process. The core layer was formed fromexemplary glass composition 1-5, which had an average CTE of 43.9×10⁻⁷°C.⁻¹. The first and second cladding layers were formed from exemplaryglass composition 2-5, which had an average CTE of 35.6×10⁻⁷° C.⁻¹. Thethickness of the glass sheet was 1.5 mm. The thickness of the core layerwas about 1.125 mm, and the thickness of each cladding layer was about0.1875 mm. The glass sheet had a surface compression of 35 MPa.

Example 2

A laminated glass sheet having the general structure shown in FIG. 1 wasformed using a fusion draw process. The core layer was formed fromexemplary glass composition 1-5, which had an average CTE of 43.9×10⁻⁷°C.⁻¹. The first and second cladding layers were formed from exemplaryglass composition 2-5, which had an average CTE of 35.6×10⁻⁷° C.⁻¹. Thethickness of the glass sheet was 0.5 mm. The thickness of the core layerwas about 0.375 mm, and the thickness of each cladding layer was about0.0625 mm. The glass sheet had a surface compression of 35 MPa.

Example 3

A laminated glass sheet having the general structure shown in FIG. 1 wasformed using a fusion draw process. The core layer was formed fromexemplary glass composition 1-7, which had an average CTE of 54.9×10⁻⁷°C.⁻¹. The first and second cladding layers were formed from exemplaryglass composition 2-5, which had an average CTE of 35.6×10⁻⁷° C.⁻¹. Thethickness of the glass sheet was 1 mm. The thickness of the core layerwas about 0.9 mm, and the thickness of each cladding layer was about0.05 mm. The glass sheet had a surface compression of 150 MPa.

Example 4

A laminated glass sheet having the general structure shown in FIG. 1 wasformed using a fusion draw process. The core layer was formed fromexemplary glass composition 1-6, which had an average CTE of 59.8×10⁻⁷°C.⁻¹. The first and second cladding layers were formed from exemplaryglass composition 2-6, which had an average CTE of 31.7×10⁻⁷° C.⁻¹. Thethickness of the glass sheet was 0.7 mm. The thickness of the core layerwas about 0.6 mm, and the thickness of each cladding layer was about0.05 mm. The glass sheet had a surface compression of 190 MPa.

Example 5

A laminated glass sheet having the general structure shown in FIG. 1 wasformed using a fusion draw process. The core layer was formed fromexemplary glass composition 1-8, which had an average CTE of 74.6×10⁷°C. The first and second cladding layers were formed from exemplary glasscomposition 2-7, which had an average CTE of 30.9×10⁷° C. The thicknessof the glass sheet was 0.7 mm. The thickness of the core layer was about0.612 mm, and the thickness of each cladding layer was about 0.044 mm.The glass sheet had a surface compression of 190 MPa.

Example 6

A laminated glass sheet having the general structure shown in FIG. 1 wasformed using a fusion draw process. The core layer was formed fromexemplary glass composition 1-11, which had an average CTE of about85×10⁻⁷° C.⁻¹. The first and second cladding layers were formed fromexemplary glass composition 2-8, which had an average CTE of about35×10⁻⁷° C.⁻¹. The thickness of the glass sheet was 0.7 mm. Thethickness of the core layer was about 0.594 mm, and the thickness ofeach cladding layer was about 0.053 mm. The glass sheet had a surfacecompression of 300 MPa.

Comparative Example 1

A non-strengthened, single layer glass sheet was formed from aboroaluminosilicate glass. The thickness of the glass sheet was 1.5 mm.

Comparative Example 2

A single layer glass sheet was formed from soda lime glass. Thethickness of the glass sheet was 0.7 mm. The glass sheet was subjectedto an ion exchange process. The ion-exchanged glass sheet had a surfacecompression of 534 MPa and a depth of layer (DOL) of 0.012 mm.

Comparative Example 3

A single layer glass sheet was formed from an ion-exchangeable glass.The thickness of the glass sheet was 0.7 mm. The glass sheet wassubjected to an ion exchange process. The ion-exchanged glass sheet hada surface compression of 855 MPa and a DOL of 0.042 mm.

Comparative Example 4

A single layer glass sheet was formed from soda lime glass. Thethickness of the glass sheet was 1.9 mm. The glass sheet was subjectedto a heat strengthening process. The heat-strengthened glass sheet had asurface compression of 30 MPa and a DOL of 0.38 mm.

Comparative Example 5

A single layer glass sheet was formed from soda lime glass. Thethickness of the glass sheet was 3.9 mm. The glass sheet was subjectedto a tempering process to form a fully tempered glass sheet.

Comparative Example 6

A single layer glass sheet was formed from an ion-exchangeable glass.The thickness of the glass sheet was 0.7 mm. The glass sheet wassubjected to an ion exchange process. The ion-exchanged glass sheet hada surface compression of 769 MPa and a DOL of 0.041 mm.

Comparative Example 7

A single layer glass sheet was formed from an ion-exchangeable glass.The thickness of the glass sheet was 0.7 mm. The glass sheet wassubjected to an ion exchange process. The ion-exchanged glass sheet hada surface compression of 915 MPa and a DOL of 0.04 mm.

Comparative Example 8

A single layer glass sheet was formed from an ion-exchangeable glass.The thickness of the glass sheet was 1 mm. The glass sheet was subjectedto an ion exchange process. The ion-exchanged glass sheet had a surfacecompression of 906 MPa and a DOL of 0.079 mm.

Comparative Example 9

A single layer glass sheet was formed from an ion-exchangeable glass.The thickness of the glass sheet was 1 mm. The glass sheet was subjectedto an ion exchange process. The ion-exchanged glass sheet had a surfacecompression of 373 MPa and a DOL of 0.114 mm.

FIGS. 3-12 illustrate various properties of the examples and comparativeexamples described herein. In FIGS. 3-10 and 12, ranges of data pointsthat were determined for multiple samples of each example or comparativeexample are shown. The values reported herein for the various propertiesare given as average values. Thus, the retained strength, indentationthreshold, Knoop scratch threshold, Vickers scratch threshold, and dropthreshold values of a given example or comparative example are reportedherein as the average values determined for that example or comparativeexample.

The surfaces of glass sheets formed according to Example 1 andComparative Example 1 were abraded with 1 mL of 90 grit SiC particlesfor 5 seconds at abrasion pressures of 5 psi, 15 psi, and 25 psi. Theretained strengths at the various abrasion pressures are shown in FIG.3.

As illustrated in FIG. 3, the laminated glass sheet retained much of itsstrength over a wide range of insult conditions as compared to thenon-strengthened glass sheet. The retained strength of the laminatedglass sheet was reduced from about 88 MPa at an abrasion pressure of 5psi to about 83 MPa at an abrasion pressure of 25 psi. Thus, the ratioof the retained strength at the abrasion pressure of 25 psi to theretained strength at the abrasion pressure of 5 psi was about 0.9. Theretained strength of the non-strengthened glass sheet was reduced fromabout 55 MPa at an abrasion pressure of 5 psi to about 46 MPa at anabrasion pressure of 25 psi. Thus, the ratio of the retained strength atthe abrasion pressure of 25 psi to the retained strength at the abrasionpressure of 5 psi was about 0.8.

The surfaces of glass sheets formed according to Example 2 andComparative Example 2 were abraded with 1 mL of 90 grit SiC particlesfor 5 seconds at abrasion pressures of 5 psi, 15 psi, and 25 psi. Theretained strengths at the various abrasion pressures are shown in FIG.4.

As illustrated in FIG. 4, the laminated glass sheet retained much of itsstrength over a wide range of insult conditions as compared to theion-exchanged soda lime glass sheet. The retained strength of thelaminated glass sheet was reduced from about 88 MPa at an abrasionpressure of 5 psi to about 83 MPa at an abrasion pressure of 25 psi.Thus, the ratio of the retained strength at the abrasion pressure of 25psi to the retained strength at the abrasion pressure of 5 psi was about0.9. The retained strength of the ion-exchanged soda lime glass sheetwas reduced from about 77 MPa at an abrasion pressure of 5 psi to about56 MPa at an abrasion pressure of 25 psi. Thus, the ratio of theretained strength at the abrasion pressure of 25 psi to the retainedstrength at the abrasion pressure of 5 psi was about 0.7. Although thecompressive stress of the ion-exchanged soda lime glass sheet (534 MPa)was significantly higher than the compressive stress of the laminatedglass sheet (35 MPa), the strength of the ion-exchanged soda lime glasssheet was reduced more sharply as the insult level increased compared tothe laminated glass sheet.

The surfaces of glass sheets formed according to Example 2 andComparative Example 3 were abraded with 1 mL of 90 grit SiC particlesfor 5 seconds at abrasion pressures of 5 psi, 15 psi, and 25 psi. Theretained strengths at the various abrasion pressures are shown in FIG.5.

As illustrated in FIG. 5, the laminated glass sheet retained much of itsstrength over a wide range of insult conditions as compared to theion-exchanged glass sheet. The retained strength of the laminated glasssheet was reduced from about 88 MPa at an abrasion pressure of 5 psi toabout 83 MPa at an abrasion pressure of 25 psi. Thus, the ratio of theretained strength at the abrasion pressure of 25 psi to the retainedstrength at the abrasion pressure of 5 psi was about 0.9. The retainedstrength of the ion-exchanged glass sheet was reduced from about 395 MPaat an abrasion pressure of 5 psi to about 53 MPa at an abrasion pressureof 25 psi. Thus, the ratio of the retained strength at the abrasionpressure of 25 psi to the retained strength at the abrasion pressure of5 psi was about 0.1. Although the compressive stress of theion-exchanged glass sheet (855 MPa) was significantly higher than thecompressive stress of the laminated glass sheet (35 MPa), the strengthof the ion-exchanged glass sheet was reduced more sharply as the insultlevel increased compared to the laminated glass sheet.

The surfaces of glass sheets formed according to Example 3, Example 4,Example 5, Example 6, Comparative Example 1, and Comparative Example 3were abraded with 1 mL of 90 grit SiC particles for 5 seconds atabrasion pressures of 5 psi, 15 psi, and 25 psi. The retained strengthsat the various abrasion pressures are shown in FIG. 6.

As illustrated in FIG. 6, the laminated glass sheets of Examples 3-6 hadhigh compressive stress compared to the non-strengthened glass sheet ofComparative Example 1 while also retaining much of their strength over awide range of insult conditions. The average strength of the laminatedglass sheet of Example 3 at the various abrasion pressures was about 171MPa. The average strength of the laminated glass sheet of Example 4 atthe various abrasion pressures was about 220 MPa. The average strengthof the laminated glass sheet of Example 5 at the various abrasionpressures was about 220 MPa. The average strength of the laminated glasssheet of Example 6 at the various abrasion pressures was about 300 MPa.Conversely, the average retained strength of the non-strengthened glasssheet at the various abrasion pressures was about 49 MPa.

As illustrated in FIG. 6, the laminated glass sheets of Examples 3-6retained much of their strength over a wide range of insult conditionsas compared to the ion-exchanged glass sheet. The retained strength ofthe laminated glass sheet of Example 3 was reduced from about 208 MPa atan abrasion pressure of 5 psi to about 117 MPa at an abrasion pressureof 25 psi. Thus, the ratio of the retained strength at the abrasionpressure of 25 psi to the retained strength at the abrasion pressure of5 psi was about 0.6. The retained strength of the laminated glass sheetof Example 4 was reduced from about 250 MPa at an abrasion pressure of 5psi to about 200 MPa at an abrasion pressure of 25 psi. Thus, the ratioof the retained strength at the abrasion pressure of 25 psi to theretained strength at the abrasion pressure of 5 psi was about 0.8. Theretained strength of the laminated glass sheet of Example 5 was reducedfrom about 270 MPa at an abrasion pressure of 5 psi to about 117 MPa atan abrasion pressure of 25 psi. Thus, the ratio of the retained strengthat the abrasion pressure of 25 psi to the retained strength at theabrasion pressure of 5 psi was about 0.4. The retained strength of thelaminated glass sheet of Example 6 was reduced from about 305 MPa at anabrasion pressure of 5 psi to about 300 MPa at an abrasion pressure of25 psi. Thus, the ratio of the retained strength at the abrasionpressure of 25 psi to the retained strength at the abrasion pressure of5 psi was about 0.98. The retained strength of the ion-exchanged glasssheet was reduced from about 395 MPa at an abrasion pressure of 5 psi toabout 53 MPa at an abrasion pressure of 25 psi. Thus, the ratio of theretained strength at the abrasion pressure of 25 psi to the retainedstrength at the abrasion pressure of 5 psi was about 0.1. Although theion-exchanged glass sheet of Example 3 had a similar average strength(178 MPa) as the laminated glass sheet (171 MPa) at the various abrasionpressures, the strength of the ion-exchanged glass sheet was reducedmore sharply as the insult level increased compared to the laminatedglass sheet. The strengths of the ion-exchanged glass sheet also wasreduced more sharply as the insult level increased compared to thelaminated glass sheets of Examples 4-6.

The surfaces of glass sheets formed according to Example 3 andComparative Example 4 were scratched at increasing scratch loads of 0 to3 N, 0 to 7 N, and 0 to 10 N. The retained strengths at the variousramped scratch loads are shown in FIG. 7. The retained strengths in FIG.7 are given as load to failure determined using the ring-on-ring testmethod.

As illustrated in FIG. 7, the laminated glass sheet retained much of itsstrength over a wide range of insult conditions as compared to theheat-strengthened glass sheet. The retained strength of the laminatedglass sheet was reduced from about 245 kgf at a ramped Knoop scratchload of 0 to 3 N to about 205 kgf at a ramped Knoop scratch load of 0 to10 N. Thus, the ratio of the retained strength at the ramped Knoopscratch load of 0 to 10 N to the retained strength at the ramped Knoopscratch load of 0 to 3 N was about 0.8. The retained strength of theheat-strengthened glass sheet was reduced from about 152 kgf at a rampedKnoop scratch load of 0 to 3 N to about 46 kgf at a ramped Knoop scratchload of 0 to 10 N. Thus, the ratio of the retained strength at theramped Knoop scratch load of 0 to 10 N to the retained strength at theramped Knoop scratch load of 0 to 3 N was about 0.3. Although theheat-strengthened glass sheet was about twice as thick as the laminatedglass sheet, the thinner laminated glass sheet retained more of itsstrength at various scratch loading conditions compared to theheat-strengthened glass sheet, which suggests that a laminated glass cansustain large flaws with relatively low thickness. This can enableproduction of thin glass articles with uniform retained strength, whichmay be beneficial for producing light weight glass sheets for use, forexample, in automotive applications and/or cover glass applications.

The surfaces of glass sheets formed according to Example 3, ComparativeExample 4, and Comparative Example 5 were scratched at increasingscratch loads of 0 to 3 N, 0 to 7 N, and 0 to 10 N. The retainedstrengths at the various ramped scratch loads are shown in FIG. 8.

As illustrated in FIG. 8, the laminated glass sheet retained much of itsstrength over a wide range of insult conditions as compared to both theheat-strengthened glass sheet and the fully-tempered glass sheet. Theretained strength of the laminated glass sheet was reduced from about668 MPa at a ramped Knoop scratch load of 0 to 3 N to about 603 MPa at aramped Knoop scratch load of 0 to 10 N. Thus, the ratio of the retainedstrength at the ramped Knoop scratch load of 0 to 10 N to the retainedstrength at the ramped Knoop scratch load of 0 to 3 N was about 0.9. Theretained strength of the heat-strengthened glass sheet was reduced fromabout 168 MPa at a ramped Knoop scratch load of 0 to 3 N to about 52 MPaat a ramped Knoop scratch load of 0 to 10 N. Thus, the ratio of theretained strength at the ramped Knoop scratch load of 0 to 10 N to theretained strength at the ramped Knoop scratch load of 0 to 3 N was about0.3. The retained strength of the fully-tempered glass sheet was reducedfrom about 195 MPa at a ramped Knoop scratch load of 0 to 3 N to about151 MPa at a ramped Knoop scratch load of 0 to 10 N. Thus, the ratio ofthe retained strength at the ramped Knoop scratch load of 0 to 10 N tothe retained strength at the ramped Knoop scratch load of 0 to 3 N wasabout 0.8. Although both the heat-strengthened glass sheet and thefully-tempered glass sheet were substantially thicker than the laminatedglass sheet, the thinner laminated glass sheet retained more of itsstrength at various scratch loading conditions compared to theheat-strengthened glass sheet and the fully-tempered glass sheet, whichfurther suggests that a laminated glass can sustain large flaws withrelatively low thickness.

FIG. 9 shows the indentation thresholds of glass sheets formed accordingto Example 2, Example 3, Comparative Example 1, Comparative Example 2,Comparative Example 3, Comparative Example 6, and Comparative Example 7.Similarly, FIG. 10 shows the indentation thresholds of glass sheetsformed according to Example 4, Example 5, Comparative Example 1,Comparative Example 2, Comparative Example 3, Comparative Example 6, andComparative Example 7. Although the laminated glass sheets of Example 2,Example 3, Example 4, and Example 5 have clad layer dimensions that aresimilar to the DOL of the ion-exchanged glass sheets of ComparativeExample 3, Comparative Example 6, and Comparative Example 7, thecompressive stresses of the laminated glass sheets are substantiallylower. Thus, conventional wisdom suggests that the laminated glasssheets would have far lower indentation thresholds compared to theion-exchanged glass sheets. However, as shown in FIG. 9, the laminatedglass sheet of Example 3 has an indentation threshold (50 N) that issimilar to that of Comparative Example 6 (55 N), despite the relativelylow compressive stress of the laminated glass sheet. Similarly, as shownin FIG. 10, the laminated glass sheet of Example 4 has an indentationthreshold (80 N) that is higher than that of Comparative Example 6 (55N), and the laminated glass sheet of Example 5 has an indentationthreshold (150 N) that is higher than those of Comparative Example 6 (55N) and Comparative Example 7 (95 N), despite the relatively lowcompressive stress values of the laminated glass sheets.

FIG. 11 shows the Knoop scratch thresholds of glass sheets formedaccording to Example 2, Example 3, Comparative Example 1, ComparativeExample 2, Comparative Example 3, Comparative Example 6, and ComparativeExample 7. Similar to the indentation threshold, conventional wisdomsuggests that the laminated glass sheets would have far lower Knoopscratch thresholds compared to the ion-exchanged glass sheets (e.g., asa result of the relatively low compressive stresses of the laminatedglass sheets). However, as shown in FIG. 11, the laminated glass sheetof Example 2 has a Knoop scratch threshold (7 N) that is greater thanthose of Comparative Example 6 (5 N) and Comparative Example 7 (4 N),and the laminated glass sheet of Example 3 has a Knoop scratch threshold(15 N) that is greater than those of Comparative Example 3 (11 N),Comparative Example 6 (5 N), and Comparative Example 7 (4 N), despitethe relatively low compressive stresses of the laminated glass sheets.Although not shown in FIG. 11, the laminated glass sheet formedaccording to Example 4 has a Knoop scratch threshold of about 18 N(within individual samples ranging between about 15 N and about 20 N).

Although not wishing to be bound by any theory, it is believed that theunexpectedly high indentation and Knoop scratch thresholds of thelaminated glass sheets may be a result of the step stress profile of thelaminated glass sheets (as opposed to the error function stress profileof the ion-exchanged glass sheets) in combination with the relativelylower magnitude of compressive stress of the laminated glass sheetscompared to the ion-exchanged glass sheets. For example, the relativelylower compressive stresses may contribute to the unexpectedly high Knoopscratch thresholds of the laminated glass sheets because highercompressive stress may encourage lateral cracks to form in the glassarticle and cause failure.

FIG. 12 shows the Vickers scratch thresholds of glass sheets formedaccording to Example 2, Example 3, Comparative Example 1, ComparativeExample 2, Comparative Example 3, and Comparative Example 7. Similar tothe indentation threshold and the Knoop scratch threshold, conventionalwisdom suggests that the laminated glass sheets would have far lowerVickers scratch thresholds compared to the ion-exchanged glass sheets(e.g., as a result of the relatively low compressive stresses of thelaminated glass sheets). However, as shown in FIG. 12, the laminatedglass sheet of Example 2 has a Vickers scratch threshold (3 N) that issubstantially the same as that of Comparative Example 1 (3 N) andgreater than those of Comparative Example 2 (1 N), Comparative Example 3(1 N), and Comparative Example 7 (1 N). The laminated glass sheet ofExample 3 has a Vickers scratch threshold (7 N) that is greater thanthose of Comparative Example 1 (3 N), Comparative Example 2 (1 N),Comparative Example 3 (1 N), and Comparative Example 7 (1 N). Thus,Example 2 and Example 3 have unexpectedly high Vickers scratchthresholds despite the relatively low compressive stresses of thelaminated glass sheets.

FIG. 13 shows the drop thresholds of glass sheets formed according toExample 3, Comparative Example 8, and Comparative Example 9. The maximumdrop height was 210 cm. The data points shown at 210 cm survived thedrop from the maximum drop height. The confidence intervals shown inFIG. 13 are 95% confidence intervals for the means. Conventional wisdomsuggests that the laminated glass sheet would have a far lower dropthreshold compared to the ion-exchanged glass sheets (e.g., as a resultof the relatively low compressive stress of the laminated glass sheet).However, as shown in FIG. 13, the laminated glass sheet of Example 3 hasa drop threshold (164 cm) that is substantially the same as that ofComparative Example 8 (168 cm) and greater than that of ComparativeExample 9 (92 cm). Thus, Example 3 has an unexpectedly high dropthreshold despite the relatively low compressive stress of the laminatedglass sheet.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the invention. Accordingly, the invention is not tobe restricted except in light of the attached claims and theirequivalents.

What is claimed is:
 1. A method comprising: forming a glass articlecomprising a core layer and a cladding layer adjacent to the core layer,the glass article comprising a coefficient of thermal expansion (CTE)mismatch between the core layer and the cladding layer and a ratio of athickness of the core layer to a thickness of the glass article suchthat a Knoop scratch threshold of the glass article is at least about 5N, an indentation threshold of the glass article is at least about 20 N,and a compressive stress of the cladding layer is at most about 500 MPa.2. The method of claim 1, wherein the ratio of the thickness of the corelayer to the thickness of the glass article is at least about 0.8. 3.The method of claim 1, wherein a retained strength of the glass articlecomprises a strength determined after abrasion of an outer surface ofthe glass article with 1 mL of 90 grit SiC particles for 5 seconds at anabrasion pressure, and a ratio of the retained strength at an abrasionpressure of 25 psi to the retained strength at an abrasion pressure of 5psi is at least about 0.3.
 4. The method of claim 3, wherein the ratioof the retained strength at the abrasion pressure of 25 psi to theretained strength at the abrasion pressure of 5 psi is at least about0.75.
 5. The method of any of claim 1, wherein the glass articlecomprises a laminated glass sheet, and the forming the glass articlecomprises forming the glass article using a fusion draw process.
 6. Aglass article comprising: a glass core layer comprising from about 50mol % to about 80 mol % SiO₂, from about 5 mol % to about 20 mol %Al₂O₃, and from about 1 mol % to about 8 mol % MgO; and a glass claddinglayer adjacent to the core layer and comprising from about 60 mol % toabout 70 mol % SiO₂, from about 6 mol % to about 18 mol % Al₂O₃, fromabout 4 mol % to about 21 mol % B₂O₃, from about 0.2 mol % to about 5mol % MgO, and from about 3 mol % to about 12 mol % CaO; wherein a Knoopscratch threshold of the glass article is at least about 5 N, and anindentation threshold of the glass article is at least about 20 N. 7.The glass article of claim 6, wherein the core layer further comprisesfrom about 3 mol % to about 11 mol % B₂O₃.
 8. The glass article of claim6, wherein the core layer further comprises from about 0.01 mol % toabout 8 mol % CaO.
 9. The glass article of any of claim 6, wherein thecore layer further comprises from about 3 mol % to about 7 mol % SrO.10. The glass article of any of claim 6, wherein the core layer furthercomprises from about 0.01 mol % to about 0.1 mol % BaO.
 11. The glassarticle of any of claim 6, wherein the cladding layer further comprisesfrom about 0.2 mol % to about 3 mol % SrO.
 12. The glass article of anyof claim 6, wherein the cladding layer further comprises from about 0.01mol % to about 2 mol % BaO.
 13. The glass article of any of claim 6,wherein the core layer comprises a coefficient of thermal expansion(CTE) of at least about 40×10⁻⁷° C.⁻¹.
 14. The glass article of any ofclaim 6, wherein the cladding layer comprises a CTE of at most about40×10⁻⁷° C.⁻¹.
 15. The glass article of claim 13, wherein the CTE of thecore layer and the CTE of the cladding layer differ by at least about5×10⁻⁷° C.⁻¹.
 16. The glass article of any of claim 13, wherein the CTEof the core layer and the CTE of the cladding layer differ by at mostabout 55×10⁻⁷° C.⁻¹.
 17. The glass article of any of claim 6, whereinthe cladding layer is substantially free of alkali metal.
 18. The glassarticle of any of claim 6, wherein a thickness of the glass article isabout 0.2 mm to about 2 mm, and a ratio of a thickness of the core layerto the thickness of the glass article is at least about 0.8.
 19. Theglass article of any of claim 6, wherein the core layer comprises atensile stress, and the cladding layer comprises a compressive stress ofat most about 500 MPa.